Problem: In math class, the girl to boy ratio is $4$ to $9$. If there are $20$ girls, how many boys are there?
Explanation: A ratio of $4$ girls to $9$ boys means that there is a group of $4$ girls for every group of $9$ boys. If there are $20$ girls, then there are $5$ groups of $4$ girls each. Because we know the ratio is $4:9$ , there must be $5$ groups of $9$ boys each. There is a total of $45$ boys in math class.